As is well known, prospecting for minerals of commercial or other value (including but not limited to hydrocarbons in liquid or gaseous form; water e.g. in aquifers; and various solids used e.g. as fuels, ores or in manufacturing) is economically an extremely important activity. For various reasons those wishing to extract such minerals from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the minerals in a geological formation and also any difficulties that may arise in the extraction of the minerals to surface locations at which they may be used.
For this reason over many decades techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before mineral extraction activities commence and also, increasingly frequently, while they are taking place.
Broadly stated, logging involves inserting a logging tool including a section sometimes called a “sonde” into a borehole or other feature penetrating a formation under investigation; and in the majority of cases using the sonde to energize the material of the rock, etc., surrounding the borehole in some way. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Such passage of the energy alters its character. Knowledge of the attributes of the emitted energy and that detected after passage through the rock may reveal considerable information about the chemistry, concentration, quantity and a host of other characteristics of minerals in the vicinity of the borehole, as well as geological aspects that influence the ease with which the target mineral material may be extracted to a surface location.
Logging techniques are employed throughout the mining industry, and also in particular in the oil and gas industries. The subject matter of the present disclosure is of benefit in logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas.
In the logging of oil, coal and gas fields (including fields combined with rock types such as shales) specific problems can arise. Broadly stated this is because it is necessary to consider a geological formation, surrounding a borehole, that typically is porous and that contains a hydrocarbon-containing fluid such as oil or gas or (commonly) a mixture of fluids only one component of which is of commercial value.
This leads to various complications associated with determining geological attributes of the oil or gas field in question. In consequence a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that as outlined above is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling. Following such deployment usually somewhat deeply into the well the logging tool is by one means or another withdrawn towards a surface location, acquiring log data as it does so on the rock surrounding the borehole.
Typically, as noted, the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question).
Depending on the logging tool design the log data acquired by the tool may be telemetered as raw signals to a surface location using a type of cable known as wireline and then processed. They may be partly or entirely processed using electronics and/or programmable devices forming part of the logging tool before being telemetered; or they can be stored in the logging tool (either as raw signals or as partly or fully processed data) for downloading after the tool has been retrieved to a surface site.
For a variety of reasons logging tools typically are elongate cylinders, of perhaps 55-200 mm in diameter. Depending on its design a logging tool may be several meters in length.
The disclosed subject matter is potentially of use in, and pertains to, all aspects of logging as described herein.
Many types of logging tool include one or more sensors that in use are pressed into contact with the wall of the borehole in which the logging tool is inserted for the purpose of acquiring log data according to methods as broadly described above.
Pressing of the sensor(s) into contact with the borehole wall (or, if present, mudcake formed on the borehole wall) is often strongly desirable in order to avoid sensor “stand-off”, i.e. a gap between the surface of the sensor and the borehole wall (or mudcake).
If stand-off exists during use of the logging tool the data recorded by the tool are likely to include information about the environment inside the borehole, as opposed to the rock surrounding it. Stand-off therefore at best is inconvenient because it is necessary to correct for it when processing logging tool signals. In some cases stand-off can be so severe as to render a set of log values essentially worthless (because of the dominance of data concerning the interior of the borehole to the virtual exclusion of useful information about the surrounding rock).
Some types of logging tool include a single sensor while some others include multiple sensors that are arrayed around the circular circumference of the logging tool cylinder. In further designs one or more sensors are mounted on deployable and retractile arms or other deployment mechanisms that selectively cause the sensors to become pressed into contact with the wall of the borehole when the logging tool has been conveyed to the correct depth for the commencement of logging. The sensors often are formed as “pads”, i.e. rigid e.g. arcuate or rectilinear parts of the logging tool that are designed for contact with the borehole wall.
A characteristic of many prior art logging tool sensors of the kinds described above is that they rely on the circularity of either the logging tool or of an arrangement of one or more arms when pressing the sensor into contact with the borehole wall. Many logging tool sensors have outer surfaces, that are intended to be pressed into contact with the borehole wall, having profiles that are circular arcs.
The use of a circular arc profile has the benefit of reducing to zero the sensor stand-off if the radius of curvature of the surface arc of the sensor as referred to above is the same as that of the borehole (and assuming that any rugosity of the borehole does not create stand-off by reason of non-circularity of the borehole wall profile).
On the other hand the stand-off will be reduced to zero only if the logging tool sensor is used in a borehole the radius of which matches the radius of curvature of the sensor surface. Such a sensor when used in a borehole of a different size will not conform to the borehole wall profile and hence will exhibit stand-off even if the sensor surface is pressed into point contact with the wall (or mudcake formed on it).
This in turn may mean that in order to eliminate the effects of stand-off over a range of borehole sizes it is necessary to stock a corresponding range of logging tools each of a different diameter (or at least of differing sensor surface radius of curvature) corresponding to a different borehole diameter. This is likely to be expensive and inefficient.
In view of the foregoing there is a need for improvements in the design of logging tool sensors.